<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>CSS</title>
    <style>
        div {
            color: aqua;
            font-size: 30px;
        }

        .red {
            color: red;
        }

        .size {
            font-size: 20px;
        }

        #blue {
            color: blue;
        }
    </style>
    <link rel="stylesheet" href="./my.css">
</head>

<body>
    <div>HTML</div>
    <p>CSS</p>
    <p>C</p>

    <p class="red">红色</p>
    <p>我不是红色</p>
    <div class="red size">div 标签</div>
    <div id="blue">div标签</div>
</body>

</html>